1. Field of the Invention
This invention relates to the field of topical fertilization with urea and particularly to improved liquid urea fertilizers and methods of topical fertilization, particularly in alkaline soils. The compositions are stable, homogeneous solutions which when topically applied to alkaline soils markedly reduce or completely eliminate urea losses by volatilization. Thus, they enable more efficient use of fertilizer nitrogen values and better control of nitrogen dosage on crops. The compositions and methods also otherwise improve soil characteristics due to the acidifying and nutrient effect of sulfuric acid.
2. Description of the Prior Art
Urea is widely used as a topical, subsurface and foliar fertilizer. Topical urea application on alkaline soils is known to result in the loss of nitrogen fertilizer values by volatilization, i.e., the ultimate conversion of urea to ammonia which escapes to the atmosphere.
Sulfuric acid has also been widely used in the agricultural industry as a soil adjuvant, a water penetration improving agent, a herbicide for a wide variety of vegetation, and as a selective herbicide on crops resistant to sulfuric acid such as onions and garlic.
Previous investigators have observed that urea, sulfuric acid, and, optionally, water can be reacted to form concentrated solutions of urea and sulfuric acid reaction products in which the urea is present as mono- and/or diurea sulfates. However, they did not recognize that this reaction can result in the formation of by-products that are toxic to plants such as sulfamic acid and ammonium sulfamate; the latter of which is a federally registered herbicide.
The urea-sulfuric acid reaction is so highly exothermic, particularly when solid urea and concentrated sulfuric acid are used as is necessary for purposes of economy, that it is difficult to control reaction temperature in the large volume production plants required to produce significant amounts of these products. In fact, it is essentially impossible to control reaction temperature and thus avoid formation of undesired by-products, during production of the higher acid content compositions having sulfuric acid/urea molar ratios greater than 0.7, with available methods. Furthermore, previous investigators did not recognize either the magnitude or importance of incipient product and/or reactant decomposition or the temperatures at which such decomposition occurs for products having different urea/sulfuric acid ratios. Their methods were not adequate to avoid incipient decomposition, particularly in the higher acid compositions, and they did not recognize the effect of such decomposition on process control or product quality.
These investigators also did not recognize that the use of urea-sulfuric acid reaction products having H.sub.2 SO.sub.4 /urea molar ratios of about 0.7 and higher markedly reduce and, in some cases, completely eliminate urea loss by volatilization even under the most adverse conditions of soil pH and ambient temperature.
D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914).
L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. (50.degree. F.) and 25.degree. C. (77.degree. F.) but, as in the case of du Toit, did not develop or disclose methods capable of handling the high heat of reaction involved in large scale industrial processing. "Ternary Systems of Urea and Acids. I Urea, Nitric Acid and Water. II. Urea, Sulfuric Acid and Water. III. Urea, Oxalic Acid and Water"; JACS, 56, 549-53 (1934).
In the article "Adding Plant Nutrient Sulfur to Fertilizer," Sulfur Institute Bulletin No. 10 (1964), the Sulfur Institute discussed the addition of nutrient sulfur to fertilizers and mentioned that urea reacts with sulfuric acid to form two complexes of urea sulfate which are useful fertilizers.
Jones, U.S. Pat. No. 4,116,664 discloses what is referred to therein as a tortuous, multistage process of producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 200.degree. F. and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred and that the products can be used as fertilizers.
Although these investigators disclosed several characteristics of urea-sulfuric acid combinations and methods of making those combinations, and that the products were useful soil adjuvants and/or fertilizers, they did not recognize that the methods they disclosed resulted in the formation of products containing toxic reaction by-products or that the use of liquid urea/sulfuric acid reaction products having H.sub.2 SO.sub.4 /urea molar ratios in excess of about 0.7 dramatically reduces or completely eliminates urea volatilization loss associated with topical application of urea on relatively alkaline soils. Those investigators did not appreciate that the incipient decomposition temperature--the temperature at which reactant and/or product decomposition commences--varies with reactant and product composition, or the effect that decomposition has on product composition.
It is therefore one object of this invention to provide improved urea-containing topical fertilizer compositions.
It is another object of this invention to provide improved urea/sulfuric acid reaction product compositions, which when applied to the soil, even in dilute form, dramatically reduce urea volatilization loss.
It is another object of this invention to provide improved topical fertilization methods employing solutions of urea-sulfuric acid reaction products.
Yet another object of this invention is to provide methods for applying urea solutions to the soil surface and minimizing or preventing the loss of nitrogen fertilizer values by volatilization loss.
Other objects, aspects, and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawings, and the appended claims: